Drilled shaft piers and driven piles are frequently used as deep foundations for buildings, bridges, and the like because they provide an economical alternative to other types of deep foundations. Drilled shaft piers are typically formed by excavating a cylindrical borehole in the ground and then placing reinforcing steel and fluid concrete in the borehole. Alternatively, pre-formed or pre-cast foundation elements, or piles, may be driven into the soil to form a foundation element. Driven piling may have a variety of cross-sectional shapes, including but not limited to round, square, or hexagonal. Driven piling may also include non-solid bodies (such as iron pipes).
The load bearing capacity of both drilled shaft piers and driven piles is a function of the end bearing capacity of the foundation element, which is determined by the maximum load that can be supported based on numerous factors, including the diameter of the element and the composition of the geomaterial (soil, rock, etc.), and the side bearing capacity of the structure, which is determined by the load capable of being borne by the skin friction developed between the side of the element and the geomaterial. The sum of the end bearing and side bearing capacities generally represents the total load that can be supported by the foundation element with acceptable foundation movement due to sinking or slippage.
It is known to use certain prior art techniques for enhancing end bearing capacity of a pier or pile in a variety of geomaterials, whether relatively hard or relatively soft. Particularly in relatively soft geomaterial, the end bearing capacity is low, adding little to the total load bearing capacity of the pier or pile, and in some instances may be discounted altogether in the calculation. End bearing capacity may also be diminished by soil disturbances at the bottom of the shaft, or by the inability to sufficiently clean out drilling detritus from the bottom of the shaft. To enhance the end bearing capacity in these and other situations, methods have been developed for pressure grouting the base or tip of the foundation element. In one of several methods known in the art, cement grout is injected under the base of concrete piers or piles after the foundation elements are in place and have cured. In successful standard tip grouting, the end bearing capacity of the foundation element is increased, although, in most methods, neither the resultant increase nor the absolute end bearing capacity of the foundation element can be directly measured.
Tip grouting may be rendered ineffective or economically prohibitive if the qualities of the particular geomaterial are such that the grout behaves unpredictably. For example, during the process of injecting pressurized grout below some foundation elements, particularly in clay, grout pressure has been observed to fall off or plateau despite the continued injection of grout. It is presumed in these situations that the grout has either begun to flow up the side of the shaft or travel through voids, cracks, hydro-fractures, seams, or solution channels in the geomaterial rather than filling the target grout area below the tip of the pier or pile. Similar difficulties are encountered when placing piers or piling in fractured rock or in gravel, each of which gives rise to uncontrolled and unpredictable grout flow. When any of the foregoing conditions occur, the pier or pile must be re-grouted or the grouting process must be abandoned. Re-grouting is a time-consuming and costly procedure because the grout lines must be flushed, the injected grout must be allowed to harden, and new grout must be pumped in. Re-grouting does not always solve the problem, however. Sometimes the target grout pressure is never reached, forcing a reevaluation of the project and potentially requiring the destruction or abandonment of the foundation element and the construction of additional or redundant piers or piles. Because each shaft can be very expensive to construct, one lost pier or pile may be devastating to a construction project.
Another disadvantage of using drilled shafts and piers as foundation elements not resolved with standard tip-grouting is the difficulty in determining the total load that can be supported by the foundation elements and the characteristics of the geomaterial at the bottom of the shafts. It is desirable to know this information in any construction project for both safety and planning purposes. In related U.S. Pat. Nos. 6,371,698, 6,869,255, and 6,942,429, the inventors disclose a method and apparatus for post-grouting that allows simple and convenient measurement of the end bearing and side bearing capacities of a foundation pier or pile while increasing the load bearing capacity of the foundation element. While this method has proved effective, it does not ensure containment of the grout during the post-grouting process because pressurized grout can rupture its enclosure at the tip of the pile or, if an enclosure is not used, pressurized grout can fracture the clay or other geomaterial, resulting in a loss of pressure.
Methods to contain grout in tip-grouting and post-grouting applications exist, but none of them have the advantage of being both (i) significantly adaptable to hold a wide range of grout volumes as on-site conditions dictate after the foundation element is put in place and (ii) capable of providing important data regarding the relationship between the geomaterial and the foundation element at the bottom of the pier or pile. For example, placing or installing an expandable “bellows”-type grout enclosure at the bottom of a shaft or pile is known to be used to contain grout in post-grouting applications. This type of grout enclosure, while somewhat adaptable, will contain only a limited range of grout volumes. Additionally, such a system provides no information on the strength of the geomaterial at the bottom of the shaft or the strain and movement associated with the geomaterial and the foundation element.
What is needed is a method and apparatus for containing grout in the target area below piers or piles in post-grouting applications that both ensures that the grout adequately provides support to the foundation element and provides data regarding the strength of the geomaterial at the bottom of the shaft and the strain and movement associated with the geomaterial and the foundation element.